High torque, low hysteresis, multiple link robot arm mechanism

Abstract

A robot arm mechanism (10) maximizes available torque and control accuracy by arranging first and second high torque motors (50, 52) in a concentric relationship about a shoulder axis (16). The first motor is coupled through a 1:1 ratio endless belt to rotate a forearm (22) about an elbow axis (32). The second motor is 1:1 directly coupled to rotate an upper arm (14) about the shoulder axis. A motor controller (100) controls the first and second motors in at least two operational states. The first operational state equally contrarotates the first and second motors to linearly extend or retract a hand (30), and the second operational state equally rotates the first and second motors to angularly displace the hand about the shoulder axis. The robot arm mechanism has a 1:1:1:2 overall drive ratio and an indexing vane (130) for eliminating positional ambiguity problems stemming from a continuous rotation capability of the robot arm mechanism. The indexing vane has four alternating short (132) and long blades (136) evenly spaced around a circle. The indexing vane is detented to rotate into four quadrant rest positions. An indexing pin (144) contacts one of the four blades for each 360 degree rotation of the first motor relative to the second motor such that the long blades break a light beam in an optical switch assembly (148) for each 720 degrees of relative rotation. The indexing vane stores and updates a rotational state of the robot arm mechanism even if the power is off and the robot arm mechanism is manually repositioned or otherwise disturbed.

We claim: 
     
       1. A multiple link robot arm apparatus, comprising: an upper arm supporting a forearm and a hand and operable for rotation about a shoulder axis, the forearm having a first end that is supported by the upper arm for rotation about an elbow axis and having a second end that supports the hand for rotation about a wrist axis;   a first motor coupled to the forearm with an X:1 drive ratio for rotatably positioning the forearm about the elbow axis, wherein X is other than 2;   a second motor coupled to the upper arm with a 1:1 drive ratio for rotatably positioning the upper arm about the shoulder axis;   a mechanical linkage operatively connecting the upper arm and the forearm, the mechanical linkage forming an active drive link between the first motor and the forearm to cause the forearm to rotate about the elbow axis in response to operation of the first motor and a passive drive link between the forearm and the hand to cause the hand to rotate about the wrist axis in response to rotation of the forearm about the elbow axis; and   a controller coordinating the operation of the first and second motors in at least a first state characterized by contrarotating the first and second motors so that the mechanical linkage causes linear displacement of the hand radial to the shoulder axis.   
     
     
       2. The apparatus of claim 1 in which the controller further coordinates the operation of the first and second motors in at least a second state characterized by rotating the first and second motors in the same direction and at equal rotational velocities so that the mechanical linkage causes angular displacement of the hand about the shoulder axis. 
     
     
       3. The apparatus of claim 1 in which X=1 and the first and second motors contrarotate at equal angular velocities. 
     
     
       4. The apparatus of claim 1 in which the hand moves at a linear displacement rate that increases as X increases. 
     
     
       5. The apparatus of claim 4 in which X is greater than 2. 
     
     
       6. The apparatus of claim 1 in which the hand moves at with a linear displacement force that increases as X decreases. 
     
     
       7. The apparatus of claim 6 in which X is less than 2. 
     
     
       8. The apparatus of claim 1 in which the hand is further operable for pointing in a predetermined pointing direction in response to a rotational state of the first motor relative to the second motor, the apparatus further including: an indexing vane coupled to the first motor for determining and storing the rotational state;   an indexing pin coupled to the second motor for changing the rotational state stored by the indexing vane when the indexing pin passes in proximity to the indexing vane; and   a sensor sensing the rotational state stored by the indexing vane and generating a rotational state signal that is received by the controller for controlling rotation of the first and second motors to move the hand in the predetermined pointing direction.   
     
     
       9. The apparatus of claim 8 in which the sensor is mounted on a stationary member and senses the rotational state stored by the indexing vane when the indexing vane is in proximity to the sensor. 
     
     
       10. The apparatus of claim 8 in which the robot arm includes powered and unpowered states and the indexing vane is capable of updating and storing the rotational state in the powered and unpowered states. 
     
     
       11. The apparatus of claim 8 in which the indexing vane is a Maltese cross-shaped device including a pair of opposed short blades positioned in quadrature to a pair of opposed long blades. 
     
     
       12. The apparatus of claim 11 in which the indexing vane is attached to a shaft that is detented to rotate to four rest positions. 
     
     
       13. The apparatus of claim 12 in which the indexing pin rotates the indexing vane to an adjacent one of the rest positions when the indexing pin contacts any of the short or long blades. 
     
     
       14. The apparatus of claim 13 in which the sensor is an optical switch assembly that generates the rotational state signal in response to a long blade of the indexing vane breaking a light beam. 
     
     
       15. The apparatus of claim 8 in which the indexing pin changes the rotational state stored by the indexing vane once for each 360 degree rotation of the first motor relative to the second motor. 
     
     
       16. In a multiple link robot arm including an upper arm supporting a forearm and a hand and operable for rotation about a shoulder axis, the forearm having a first end that is supported by the upper arm for rotation about an elbow axis and having a second end that supports the hand for rotation about a wrist axis, a method of controlling the robot arm, comprising: coupling a first motor to the forearm with an X:1 drive ratio for rotatably positioning the forearm about the elbow axis, wherein X is other than 2;   coupling a second motor to the upper arm with a 1:1 drive ratio for rotatably positioning the upper arm about the shoulder axis;   connecting the upper arm to the forearm with a mechanical linkage that forms an active drive link between the first motor and the forearm to cause the forearm to rotate about the elbow axis in response to operation of the first motor and a passive drive link between the forearm and the hand to cause the hand to rotate about the wrist axis in response to rotation of the forearm about the elbow axis; and   coordinating the operation of the first and second motors in at least a first state characterized by contrarotating the first and second motors so that the mechanical linkage causes linear displacement of the hand radial to the shoulder axis.   
     
     
       17. The method of claim 16 further including the operation of the first and second motors in at least a second state characterized by rotating the first and second motors in the same direction and at equal rotational velocities so that the mechanical linkage causes angular displacement of the hand about the shoulder axis. 
     
     
       18. The method of claim 16 in which X=1 and the first and second motors contrarotate at equal angular velocities. 
     
     
       19. The method of claim 16 in which X is greater than 2, and the hand moves at a linear displacement rate that increases as X increases. 
     
     
       20. The method of claim 16 in which X is less than 2, and the hand moves at with a linear displacement force that increases as X decreases.